Rapid prototyping apparatus

ABSTRACT

Apparatus for producing an object by sequentially forming thin layers of a construction material one on top of the other responsive to data defining the object, the apparatus comprising: a plurality of printing heads each having a surface formed with a plurality of output orifices and controllable to dispense the construction material through each orifice independently of the other orifices; a shuttle to which the printing heads are mounted; a support surface; and a controller adapted to control the shuttle to move back and forth over the support surface and as the shuttle moves to control the printing heads to dispense the construction material through each of their respective orifices responsive to the data to form a first layer on the support surface and thereafter, sequentially the other layers; wherein each printing head is dismountable from the shuttle and replaceable independently of the other printing heads.

RELATED APPLICATIONS

The present application claims benefit of U.S. Provisional Application60/466,731 filed May 1, 2003, the disclosure of which is incorporatedherein by reference.

FIELD OF THE INVENTION

The present invention relates to apparatus, hereinafter “rapidproduction apparatus”, for producing a 3-dimensional object bysequentially forming thin layers of material one on top of the other,responsive to data defining the object.

BACKGROUND OF THE INVENTION

Rapid production apparatus (RPAs) form objects by sequentially formingthin layers, hereinafter “construction layers”, of a material one on topof the other responsive to data, hereinafter “construction data”,defining the objects. There are numerous and varied types of RPAs anddifferent methods by which they form the thin construction layers theyuse to build an object.

One type of RPA, conventionally referred to as an “ink-jet RPA”,“prints” each layer of an object it builds. To form a given layer theink-jet RPA controls. at least one dispenser, referred to as a “printinghead”, to dispense at least one construction material in liquid form ina pattern responsive to construction data for the object and thensolidifies the dispensed material. At least one construction material,hereinafter a “building material” (BM), dispensed to form the layer isprinted in the shape of a cross section of the object. Building materialin adjacent construction layers is printed in the shape of thin crosssections of the object that are displaced relative to each other by asmall incremental distance along a same direction, hereinafter referredto as a “stacking direction”, relative to the object.

For convenience of exposition, the cross sections of the object in whoseshapes the construction layers are formed are assumed to be parallel tothe xy-plane of a suitable coordinate system and the stacking directionis in the z-direction of the coordinate system. Optionally, the buildingmaterial is a photopolymer, which is hardened after deposition byexposure to suitable electromagnetic radiation, typically UV radiation.

For many construction objects, because of the complexity and/or shape ofthe objects, construction layers comprising only BM printed in the shapeof cross sections of the construction objects are not completelyself-supporting and require support during construction of the object.For such cases, at least one construction material, hereinafter referredto as “support material” (SM), is printed as required in suitableregions of each layer to provide support for the building material inthe layer. The support material and/or a shape in which it is formed, issuch that upon completion of the object it can be removed from theobject without substantially damaging the building material. In someembodiments, the support material, like the building material, is also aphotopolymer.

An ink-jet type of RPA typically comprises at least one ink-jet printinghead comprised in a “printing head block”, which is mounted to a“shuttle”. Each printing head has an array of one or more outputorifices and is controllable to dispense construction material from eachorifice independently of dispensing construction material from the otherorifices. The construction material comprises one or more types ofphotopolymer materials typically stored in at least one cartridge fromwhich a suitable configuration of pipes transports the material ormaterials to one or more reservoirs in the printing head block fromwhich the printing head receives the material . Optionally, to maintainappropriate viscosity of the at least one photopolymer, a controllercontrols at least one heater, optionally mounted to the printing block,print head and/or reservoir, to heat the photopolymer to a suitableoperating temperature. The one or more types of photopolymers may,generally, be dispensed in any combination, separately or together,simultaneously or consecutively.

During construction of an object, a controller controls the shuttle torepeatedly move over a support surface, hereinafter a “constructionplatform”, parallel to the xy-plane. As the shuttle moves, thecontroller controls each printing head to dispense construction materialselectively through its orifices responsive to construction datadefining the object to print the construction layers from which theobject is made on the construction platform, one layer after the other,one on top of the other. Mounted to the shuttle, adjacent to theprinting head block are one or more sources of electromagneticradiation, optionally UV radiation, for curing the photopolymerconstruction material printed in each construction layer. Also,optionally, mounted to the shuttle adjacent to the at least one printinghead block is a “leveling roller” which levels newly printed layers ofconstruction material to a predetermined layer height by removingsurplus material and/or peaks of material in the layer. The surplusmaterial removed from the layer is wiped off the roller by a “cleaningwiper” and gathered in a waste container comprised in the shuttle.

Optionally, in moving the shuttle over the support surface duringproduction of a construction layer, the controller controls the shuttleto move back and forth along the x-direction. Optionally, at any one ormore reversals of the shuttle along the x-direction the controllerincrements displacement of the shuttle in the y-direction. Followingproduction of a given construction layer, either the constructionplatform is lowered or the shuttle raised, along the stacking directionby a distance equal to a thickness of a next construction layer to beproduced over the just formed given layer.

During construction of an object, excess cured photopolymer constructionmaterial has a tendency to accumulate on or between the at least oneprinting head in the printing head block and on the cleaning wiper. Theaccumulated material may result in total or partial blockage of outputorifices, generating inaccuracies in deposition of construction materialand/or damage to a printed layer as the printing heads and roller moveover a printed layer. Often, functioning of a printing head block may beso degraded by accumulated photopolymer “debris” that the printing blockmust be replaced. Replacing a printing head block is generallyexpensive, time consuming, and requires recalibration of the RPA so thatdeposition of polymer via the output orifices can be accuratelycontrolled.

Configurations of ink-jet type RPAs are described in U.S. Pat. No.6,259,962, U.S. Pat. No. 6,658,314, U.S. Pat. No. 6,569,373 and U.S.application Ser. Nos. 10/101,089, 09,484,272, 10/336,032, thedisclosures of which are incorporated herein by reference.

SUMMARY OF THE INVENTION

An aspect of some embodiments of the invention relates to providing anink-jet type rapid production apparatus (RPA) having improvedoperational characteristics.

An aspect of some embodiments of the invention relates to providing anRPA comprising a shuttle having a printing head block for which eachprinting head therein is dismountable and replaceable independently ofthe other printing heads in the block.

In accordance with an embodiment of the invention, the shuttle andprinting heads are configured so that when a printing head is replacedit is automatically aligned by alignment structures comprised in theprinting head block and the printing head. In accordance with anembodiment of the invention the printing head is associated with amemory comprising profile data that specifies operating characteristicsof the printing head that is used by a controller in the RPA to controlthe printing head. In some embodiments of the invention the memory iscomprised in the printing head.

An aspect of some embodiments of the invention relates to providing anRPA having an improved lamp that provides radiation for curingphotopolymer construction materials.

Photopolymer construction material along edges of construction layersformed by an RPA is often poorly polymerized resulting in edges thatsometimes have relatively poor definition and may remain soft andsticky. A radiation lamp, in accordance with an embodiment of theinvention provides a relatively large portion of its radiant energy sothat the radiation is incident on construction layers at relativelylarge angles to a normal to their planes. The large incident angleradiation is relatively more efficient in polymerizing material alongedges of a construction layer than radiation that is incident atrelatively small angles. An aspect of some embodiments of the inventionrelates to providing an RPA having an improved cleaning wiper forremoving photopolymer debris that accumulates on surfaces of the RPA.

There is therefore provide in accordance with an embodiment of theinvention apparatus for producing an object by sequentially forming thinlayers of a construction material one on top of the other responsive todata defining the object, the apparatus comprising:

a plurality of printing heads each having a surface formed with aplurality of output orifices and controllable to dispense theconstruction material through each orifice independently of the otherorifices;

a shuttle to which the printing heads are mounted;

a support surface; and

a controller adapted to control the shuttle to move back and forth overthe support surface and as the shuttle moves to control the printingheads to dispense the construction material through each of theirrespective orifices responsive to the data to form a first layer on thesupport surface and thereafter, sequentially the other layers; wherein

each printing head is dismountable from the shuttle and replaceableindependently of the other printing heads.

Optionally, each printing head comprises at least one registrationstructure that matches a registration structure comprised in the shuttleand when a printing head is mounted to the shuttle its at least oneregistration structure contacts the corresponding shuttle registrationstructure and positions the printing head accurately in the shuttle.Optionally, the orifices in each printing head are equally spaced in alinear array having a first orifice located at a first end of the array.Optionally, the at least one registration structure comprised in eachprinting head and its corresponding shuttle registration structureposition the printing heads so that their respective lines of orificesare parallel. Optionally, wherein the lines of orifices are arrayedalong a direction perpendicular to the lines of orifices. Optionally,the at least one registration structure comprised in each printing headand its corresponding shuttle registration structure, position theprinting heads so that the first orifice in each printing head isaccurately positioned relative to the first orifices of the otherprinting heads. Optionally, projections on the support surface ofparallel lines through the centers of the orifices that areperpendicular to the lines of orifices are substantially equally spacedone from the other. Optionally, distances of the first orifices from asame plane perpendicular to the lines of orifices are located atdistances from the plane in accordance with an expression of the formy(n)=C+n(d_(y)/N), where y is the distance from the plane, C is aconstant, N is a number of printing heads, d_(y) is a distance betweenadjacent orifices in a same printing head and for each of the printingheads, n is a different integer satisfying 0≦n≦(N−1). Optionally, thecontroller controls the shuttle to move along a direction perpendicularto the lines of orifices when construction material is dispensed fromorifices in the printing heads during formation of a layer. Optionally,the distances y(n) are such that a printing head deposits droplets on agiven line in the layer parallel to the lines of orifices at locationssuch that the droplets are substantially not contiguous with anydroplets of material deposited previously on the given line by other ofthe N printing heads. Optionally, each droplet deposited between twoclosest, previously deposited droplets on the given line, is equidistantfrom the two previously deposited droplets

In some embodiments of the invention, the at least one registrationstructure in each printing head comprises at least one registration pinthat protrudes from the printing head and has an end accuratelypositioned relative to the line of orifices.

Optionally, the corresponding shuttle registration structure is asurface and wherein the registration pin and registration surface arepositioned so that when the printing head is mounted to the shuttle thetip of the pin butts up against the surface. Alternatively oradditionally, the at least one registration pin comprises threeregistration pins. Optionally, a line between the tips of two of theregistration pins is accurately parallel to the line of orifices.Optionally, the tip of a third registration pin is displaced parallel tothe line of orifices and away from all the orifices by an accuratedistance relative to the first orifice.

In some embodiments of the invention, each printing head is associatedwith a memory. Optionally, the memory is comprised in the printing head.Additionally or alternatively, the memory comprises profile data thatspecifies operating characteristics peculiar to the printing head thatthe controller uses to control the printing head. Optionally, theprofile data becomes accessible to the controller automatically when theprinting head is mounted to the shuttle. Additionally or alternatively,each orifice is associated with its own actuator controllable to controldispensing of the construction material from the orifice and wherein theprofile data comprises data useable to control the actuator.

In some embodiments of the invention, the apparatus comprises atemperature monitor that generates signals responsive to temperature ofthe printing head. Optionally, the memory comprises calibration datathat correlates a characteristic of the signals with temperature of theprinting head.

In some embodiments of the invention, the printing head comprises a heatsource controllable to maintain the printing head at a desiredtemperature and wherein the memory comprises data useable to control theheat source.

In some embodiments of the invention, the memory comprises data useableto determine the position of the orifices relative to the orifices ofother printing heads mounted to the shuttle.

In some embodiments of the invention, the construction materialcomprises a photopolymer. Optionally, the apparatus comprises a lampthat provides radiation to polymerize the photopolymer. Optionally, thelamp provides a substantial portion of the radiation so that it isincident on the layers at substantially non-normal angles to theirplanes.

There is further provided, in accordance with an embodiment of theinvention, apparatus for producing an object by sequentially formingthin layers of a material one on top of the other responsive to datadefining the object, the apparatus comprising:

at least one printing head having a surface formed with at least oneoutput orifice and controllable to dispense a photopolymer material inliquid form through the orifice;

a lamp controllable to provide radiation that polymerizes thephotopolymer; and

a controller adapted to control the printing head to dispense thephotopolymer and sequentially form the layers and the lamp to irradiateand polymerize the dispensed photopolymer; wherein

a substantial portion of radiation provided by the lamp is directed sothat it is incident at a substantially non-normal angle on the layers.

Optionally, the lamp comprises a radiation source and a reflector thatreflects light provided by the source so that it is incident at asubstantially non-normal angle on the layers. Additionally oralternatively, the magnitude of the angle is greater than 20° relativeto the normal to the layers. In some embodiments of the invention, themagnitude of the angle is greater than about 30° relative to the normal.In some embodiments of the invention, the magnitude of the angle isequal to about 45° relative to the normal.

In some embodiments of the invention, the reflector comprises at leastone parabolic reflector and at least a portion of the light source islocated at the focus of the reflector. Optionally, the reflector is apolygonal reflector that approximates a parabolic reflector. Optionally,the angle of incidence is positive for a portion of the light andnegative for a portion of the light.

In some embodiments of the invention, the radiation source is adischarge type bulb. Optionally, the bulb is an Hg or Xe discharge bulb.

In some embodiments of the invention, the lamp comprises LEDscontrollable to provide the radiation that polymerizes the photopolymer.

There is further provided in accordance with an embodiment of theinvention, Apparatus for producing an object by sequentially formingthin layers of a material one on top of the other responsive to datadefining the object, the apparatus comprising:

at least one printing head controllable to dispense a photopolymermaterial in liquid form;

a lamp controllable to provide radiation that polymerizes thephotopolymer; and

a controller adapted to control the printing head to dispense thephotopolymer and sequentially form the layers and the lamp to irradiateand polymerize the dispensed photopolymer; wherein the lamp comprises anarray of LEDs controllable to provide the radiation that polymerizes thephotopolymer.

Optionally the apparatus comprises a microlens that configures lightfrom the LED into a cone beam of radiation having a relatively largecone angle. Optionally, the cone angle is larger than about 80°.Optionally, the cone angle is larger than about 100°.

In some embodiments of the invention, the array of LEDs is locatedrelatively far from the layers and comprising a radiation conductor foreach LED in the array that pipes radiation from the LED to a locationrelatively close to the layers from which the radiation illuminatesregions of the layers.

In some embodiments of the invention, the controller controlsintensities of UV light provided by LEDs in the array independently ofintensities provided by other LEDs in the array.

In some embodiments of the invention, the controller turns on and offLEDs in the array so as to reduce radiation from the array that is noteffective in polymerizing photopolymer in the layers.

In some embodiments of the invention, the apparatus comprises a wiperand wherein the controller is adapted to move at least one printing headover the wiper to clean the surface in which the orifices are formed.

There is further provided in accordance with an embodiment of theinvention, apparatus for producing an object by sequentially formingthin layers of a material one on top of the other responsive to datadefining the object, the apparatus comprising:

at least one printing head having a surface formed with at least oneoutput orifice and controllable to dispense a photopolymer material inliquid form through the orifice;

a wiper; and

a controller adapted to control the printing head to dispense thephotopolymer and sequentially form the layers move the printing headover the wiper to clean the surface in which the orifices are formed.

Additionally, or alternatively, the wiper comprises at least onecleaning blade having an edge that scrapes excess construction materialfrom the surface when the controller controls the surface to move overthe wiper.

Optionally, the edge of at least one cleaning blade contacts the surfacewhen the surfaces move over the wiper.

Optionally, the cleaning blade is formed from a resilient material sothat the edge that contacts the surfaces resiliently contacts thesurface. Optionally, the edge is scalloped and has a different scallopcorresponding to each printing head of the at least one printing head.

In some embodiments of the invention, the at least one printing headcomprises a plurality of printing heads.

Optionally, the cleaning blade is formed with at least one slot thatpartitions the cleaning blade into a plurality of teeth each having anedge that contacts an orifice surface of a different one of theplurality of printing heads and scrapes excess construction materialform the surface.

Additionally or alternatively, wherein the at least one cleaning bladecomprises at least two cleaning blades. Optionally, a cleaning blade ofthe at least two cleaning blades has an edge that does not contact theorifice surface of a printing head but moves along and in closeproximity to the surface when the controller controls the surface tomove over the wiper. Optionally, when the surface moves over the wiper,regions of the surface move over the edge that does not contact thesurface prior to contacting the edge that contacts the surfaces.

In some embodiments of the invention, the apparatus comprises anobstacle detection system that detects defects in a layer that protrudefrom a surface of the layer. Optionally, the obstacle detection systemcomprises: a laser that provides a laser beam that contacts or islocated close to the surface of the layer along a length of the laserbeam; and a detector that receives light from the laser beam; whereinlight that the detector receives from the laser is at least partiallyblocked by a defect that protrudes from the surface.

An aspect of some embodiments of the invention relates to providing newconstruction materials for use in a jet-ink RPA, which when used toconstruct an object results in the object having improved structuralstrength relative to that which it would have if produced using priorart inkjet construction materials.

BRIEF DESCRIPTION OF FIGURES

Non-limiting examples of embodiments of the present invention aredescribed below with reference to figures attached hereto, which arelisted following this paragraph. In the figures, identical structures,elements or parts that appear in more than one figure are generallylabeled with a same numeral in all the figures in which they appear.Dimensions of components and features shown in the figures are chosenfor convenience and clarity of presentation and are not necessarilyshown to scale.

FIG. 1 schematically shows a rapid production apparatus (RPA) inaccordance with an embodiment of the present invention;

FIG. 2A schematically shows a bottom perspective view of a shuttle,which is comprised in the RPA shown in FIG. 1 and has individuallyreplaceable printing heads, and, in accordance with an embodiment of theinvention;

FIG. 2B schematically shows a bottom view of the shuttle show in FIG.2A;

FIG. 2C schematically shows the shuttle in FIGS. 2A-2B with its printingheads removed;

FIG. 2D schematically shows a printing head, in accordance with anembodiment of the invention;

FIGS. 2E and 2F schematically show perspective and cross section viewsrespectively of a system for providing construction material to printingheads in an RPA, in accordance with an embodiment of the presentinvention;

FIG. 3A schematically illustrates lines along which different outputorifices of the printing head dispense construction material to form aconstruction layer of an object, in accordance with an embodiment of theinvention;

FIG. 3B shows a portion of FIG. 3A enlarged for convenience ofpresentation;

FIG. 4A schematically illustrates a method of dispensing constructionmaterial to produce a construction layer, in accordance with prior art;

FIG. 4B schematically illustrates a method of dispensing constructionmaterial to produce a construction layer, in accordance with anembodiment of the present invention;

FIG. 4C schematically shows a shuttle configured to dispenseconstruction material as illustrated in FIG. 4B, in accordance with anembodiment of the present invention;

FIG. 5 schematically shows a bottom view of another shuttle, inaccordance with an embodiment of the present invention;

FIGS. 6A-6C schematically show a perspective partially cutaway view andcross sectional views respectively of a lamp that provides UV light forpolymerizing construction material, in accordance with an embodiment ofthe invention;

FIG. 6D shows a graph that graphs relative intensity of light from a UVlamp that is reflected from a construction layer being formed by the RPAshown in FIG. 1 as a function of height above the layer of the aperturethrough which the lamp provides the light;

FIGS. 7A and 7B show schematic cross sectional views of other UV lamps,in accordance with embodiments of the present invention;

FIG. 8 schematically shows UV lamps comprising LEDs, for providingpolymerizing UV light, in accordance with an embodiment of the presentinvention;

FIGS. 9A and 9B schematically show perspective views of a shuttle havingUV lamps that comprise LEDs that are located relatively far fromconstruction layers that the shuttle is controlled to form, inaccordance with an embodiment of the present invention;

FIGS. 10A and 10B schematically show a perspective and cross sectionalview respectively of a shuttle undergoing maintenance cleaning inaccordance with an embodiment of the invention;

FIGS. 10C-10D schematically show variations of cleaning blades used toclean a shuttle, in accordance with an embodiment of the invention;

FIGS. 11A and 11B schematically show perspective and cross section viewsof another cleaning blade configuration, in accordance with anembodiment of the invention;

FIG. 11C schematically shows a perspective view of a variation of thecleaning blade shown in FIGS. 11A and 11B, in accordance with anembodiment of the invention;

FIG. 12A schematically shows a system for detecting protuberances on aconstruction layer formed by an RPA, in accordance with an embodiment ofthe present invention;

FIGS. 12B and 12C show schematic cross sections of the system shown inFIG. 12A;

FIG. 12D schematically shows a variation of the system shown in FIG.12A, in accordance with an embodiment of the present invention;

FIG. 13 shows a schematic graph illustrating interdependence ofparameters that characterize performance of an RPA, in accordance withan embodiment of the invention; and

FIG. 14 schematically shows a method of producing a relatively thinconstruction layer having relatively high printing resolution, inaccordance with an embodiment of the present invention.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

FIG. 1 schematically shows an ink-jet RPA 20 producing an object 22 on aconstruction platform 24, in accordance with an embodiment of thepresent invention. RPA 20 comprises a controller 26 and a shuttle 28comprising a printing head block 50, a leveling roller 27 and,optionally, two sources 120 of radiation suitable for polymerizingphotopolymers used by the RPA to construct objects, in accordance withan embodiment of the invention. Optionally, construction platform 24 ismounted to a worktable 25 and is controllable to be lowered and raisedwith respect to the worktable.

Periodically, during production of object 22, RPA 20 controller 26 movesshuttle 28 to a maintenance area 220 on worktable 25 comprising a sump222 and at least one cleaning blade. By way of example, maintenance area220 comprises two cleaning blades, a first cleaning blade 225 and asecond cleaning blade 227. At maintenance area 220 controller 26performs a cleaning procedure to remove waste construction material,“debris”, that may accumulate on printing heads comprised in printinghead block 50. Maintenance area 220 and the cleaning procedure isdiscussed below.

For simplicity, it is assumed that photopolymers used by RPA 20 are UVcurable and that radiation sources 120 are UV lamps. RPA 20 is shownvery schematically and only features and components of the RPA germaneto the discussion are shown in FIG. 1. A coordinate system 21 is used toreference locations and positions of features and components of RPA 20.

To produce object 22, controller 26 controls shuttle 28 to move back andforth over construction platform 24, optionally, parallel to the x-axisin directions indicated by a double headed block arrow 31. Following oneor more reversals of direction along the x-axis, the controller mayadvance shuttle 28 by an incremental distance, optionally, parallel tothe y-axis along a direction indicated by block arrow 32. As shuttle 28moves over construction platform 24 controller 26 controls the printingheads responsive to construction data that defines object 22, todispense construction material (building material, BM, and/or supportmaterial, SM, as required) and form construction layers 34 that are usedto produce the object.

After construction material is freshly printed to form a region of agiven construction layer 34, leveling roller 27 contacts the region, andflattens and levels it to a desired thickness by shaving off an upperportion of the printed material. To achieve the shaving action, roller27 rotates in a direction that it would rotate were it rolling on theconstruction layer in a direction along which shuttle 28 advances, butat a speed of rotation greater than that which corresponds to the linearspeed of advance of the shuttle. A suitable wiper and waste material“catchment” (not shown) mounted in shuttle 28 cleans waste constructionmaterial from roller 27.

Construction layers 34 are stacked in a direction, i.e. a stackingdirection, perpendicular to construction platform 24, parallel to thez-axis. Following formation of a given construction layer 34,optionally, construction platform 24 is lowered by a distancesubstantially equal to a thickness of a next construction layer to beformed on the given construction layer. For convenience of presentation,thickness of construction layers 34 is greatly exaggerated in FIG. 1.

By way of example, object 22 is a copy of a vase 36 shown in an inset 38and is shown on construction platform 24 partially constructed. Vase 36is schematically shown formed from “data cross section” layers 40 thatare defined by the vase's construction data. A block arrow 42schematically indicates that the construction data is input to and/orgenerated responsive to appropriate input data, in controller 26 andsuitably formatted to control production of construction layers 34.

FIG. 2A schematically shows shuttle 28 in a perspective view as seenfrom the bottom of the shuttle. From the perspective of FIG. 2Acoordinate system 21 has its x-axis and its z-axis inverted with respectto the directions of these axes shown in FIG. 1.

Printing head block 50 is optionally formed with a plurality of sockets51, each of which is adapted to receive a printing head 52 that may beinserted and removed from the socket independently of having to insertor remove a printing head from others of the sockets. Sockets 51 aremore clearly shown in FIGS. 2B and 2C, which show shuttle 28 as seenfrom the bottom, respectively with and without printing heads 52inserted into the sockets. FIG. 2D schematically shows a printing head52 in accordance with an embodiment of the invention, by itself, inwhich details of the printing head are more clearly shown than in FIGS.2A-2C.

By way of example, block 50 comprises eight sockets 51. Optionally,different printing heads 52 or different groups of printing heads 52 arededicated to printing different construction materials. For example,some of printing heads 52 may be dedicated to printing only BM or aparticular type of BM, while other printing heads 52 may be dedicated toprinting only SM or a particular type of SM. Printing heads 52 may bedesignated and configured as BM or SM dedicated printing headssubstantially in any manner. For example, a number of printing heads 52dedicated to printing BM may be different from a number of printingheads 52 dedicated to printing SM. Additionally or alternatively,adjacent printing heads 52 may be dedicated to printing differentconstruction materials, one to printing BM and the other to printing SM.

By way of example, in printing head 50, a group of four printing heads52 inserted into sockets 51 indicated by bracket 54 are assumed to bededicated to printing BM and a group of four printing heads 52 insertedinto sockets 51 indicated by a bracket 53 are assumed dedicated toprinting SM. Where convenience warrants, sockets 51 indicated by bracket53 are also referred to as sockets 53 and sockets 51 indicated bybracket 54 are also referred to as sockets 54.

Printing head block 50 and printing heads 52 are configured, inaccordance with an embodiment of the invention, so that each printinghead may be relatively easily replaced, for example, as may be requiredbecause of damage or as indicated by a service regimen. Optionally, allprinting heads 52 are substantially the same.

Each printing head 52 comprises a housing 56, most clearly shown in FIG.2D, formed with a plurality of collinear, optionally equally spacedoutput orifices 58 through which construction material is dispensed. Forconvenience a dashed line 59 shown in FIG. 2D, and shown for someprinting heads 52 in FIGS. 2A and 2B, indicates a line along whichcollinear orifices 58 are arrayed. Description of methods and devicesfor providing construction material to printing heads 52, in accordancewith an embodiment of the invention, are given below in the discussionof FIGS. 2E and 2F.

A circuit board 55 comprises circuitry 57 for controlling piezoelectricactuators (not shown) comprised in housing 56 that are actuated todispense construction material through orifices 58 and other componentsof printing head 52. Connectors 47 connect circuit board 55 to circuitryin printing head block 28 that connects to controller 26 (FIG. 1). Inaccordance with an embodiment of the invention, circuit board 55comprises a memory 49 having data, “profile data” that specifiesoperating characteristics of printing head 52. Profile data optionallycomprised in memory 49 is discussed below.

Printing heads 52 and printing head block 50 comprise correspondingalignment features. Some of the alignment features cooperate toautomatically align a printing head 52 when the printing head isinserted into any one of sockets 51 so that its line 59 of outputorifices 58 is parallel to a same line, which is, optionally, they-axis. Lines 59 of orifices 58 in all printing heads 52 mounted toprinting head block 50 are therefore parallel to each other to arelatively high degree of accuracy. Optionally, lines 59 of orifices 58in printing heads 52 are equally spaced one from the other.

Some of the corresponding alignment features cooperate to align printingheads 52 so that, optionally, the y-coordinates of orifices in differentprinting heads dedicated to print a same construction material aredifferent. For example, in accordance with an embodiment of theinvention, the y-coordinates of orifices 58 in different printing heads52 inserted in sockets 53 (i.e. sockets 51 indicated by bracket 53) aredifferent. Similarly, whereas the y-coordinates of orifices 58 in aprinting head 52 inserted into a socket 54 (i.e. a socket 51 indicatedby bracket 54) may be the same as the y-coordinates of orifices in aprinting head 52 inserted into a socket 53, the y-coordinates oforifices 58 in two different printing heads 52 in sockets 54, aredifferent. Optionally, the configuration of printing heads in sockets 54is the same as that of printing heads in sockets 53 and the discussionbelow, while referring to printing heads in sockets 54, is understoodto, optionally, apply to printing heads in sockets 53.

Let a first orifice 58 in each printing head 52 be an orifice closest tothe xz plane (FIG. 2A) and let a distance between adjacent orifices in asame printing head be “d_(y)”. Optionally, the y-coordinate of the firstorifice in each printing head 52 located in a socket 54 has a valuegiven by an expression of the form

y=C+n(d _(y)/N)=C+nΔd _(y)  1)

where N is a number of sockets 54, Δd_(y)=d_(y)/N and for each socket nis a different integer satisfying the condition 0≦n≦(N−1).

Optionally, the alignment features comprise for each printing head 52,two x alignment pins 60 and a y alignment pin 62 (most clearly shown inFIGS. 2B and 2D). Optionally, each x pin has a rounded end having a tip61 and each y pin 62 has a rounded end having-a tip 63. Tip 61 of each xpin 60 is displaced by a same accurate distance Δx along the x-axisrelative to the x-coordinate of line 59. Optionally, Δx is substantiallythe same for all printing heads 52. Tip 63 of y pin 62 is displaced, byan accurate distance Δy along the y-axis from the y-coordinate of thefirst orifice of printing head 52. Optionally, Δy is substantially thesame for all printing heads 52.

Each socket 54 comprises two x-alignment buttons 64 and a y alignmentbutton 66 corresponding respectively to x alignment pins 60 and yalignment pin 62 comprised in each printing head 52. X alignment buttons64 are not shown in FIG. 2A but are schematically shown in FIG. 2B andmost clearly in FIG. 2C. Each x alignment button 64 has a sameaccurately controlled length and ends in a planar “alignment surface”65. Each socket 54 comprises at least one resilient element 68, such asa leaf or coil spring. When a printing head 52 is inserted into socket54 the at least one resilient element 56 presses the printing head sothat tips 61 of its x alignment pins 60 contact x alignment surfaces 65of alignment buttons 64 in the socket. The configuration of x alignmentpins 60 and buttons 64 result in lines 59 of orifices 58 of printingheads 52 inserted into sockets 51 being relatively accurately parallel.

Each y-button 66 comprised in sockets 54 has a different length,optionally given by equation 1, and ends in a planar alignment surface67. A resilient element 69 comprised in each socket 54 resiliently urgesa printing head 52 inserted into the socket so that the printing head'sy alignment tip 63 presses against y-alignment surface 67 of thealignment button comprised in the socket. The configuration of yalignment pins 62 and buttons 66 provide that the first orifice 58 ofeach printing head 52 inserted into a different one of sockets 54 has adifferent y-coordinate, optionally given by equation 1. Orifices 58 ofeach printing head 52 in a socket 54 are thereby displaced relative tothe orifices of the other printing heads in sockets 54 by a differentmultiple of (d_(y)/N)=Δd_(y). Projections of orifices 58 from allprinting heads 52 in sockets 54 onto a line parallel to the y-axis areequally spaced along the line by a distance equal to Δd_(y). By way ofexample, for the configuration of sockets 54 and y alignment buttons 66shown in FIG. 1, displacement of printing heads 54 along the y-axisdecreases linearly with increase of the x-coordinate of the printingheads relative to the x-coordinate of fixed feature of printing headblock 50.

FIG. 2E schematically shows a perspective view of printing head block 50right side up and printing heads 52 mounted in the block connected toreservoirs 401, 402, 403 and 404 comprised in the printing head blockthat store construction material provided to the printing heads. Theprinting heads and reservoirs are shown as if seen through the printinghead block which is shown in dashed lines. FIG. 2F schematically shows across section view of a printing head 52 shown in FIG. 2E.

Printing heads 52 that are located in sockets 54 (FIGS. 2B and 2C) and,optionally, print building material (BM) are indicted by a bracketlabeled “BM” and will be referred to as BM printing heads. Each BMprinting head 52 is coupled to reservoirs 401 and 403 that store BM andprovide BM to the printing heads. A supply line 409 connects reservoir401 to a “supply” pump (not shown) that pumps BM to reservoir 401,optionally, from a BM supply cartridge, generally located at a distancefrom printing head block 50. A reflux safety valve 411 optionallyconnects reservoir 403 to a vacuum pump (not shown) that maintains aslight vacuum in reservoirs 401 and 402.

Similarly, a bracket labeled “SM” indicates printing heads 52 that arelocated in sockets 53 and, optionally, print support material (SM), andwill be referred to as SM printing heads. Each SM printing head 52 iscoupled to reservoirs 402 and 404 that store SM and provide SM to the SMprinting heads. A supply line 410 connects reservoir 402 to a pump thatpumps SM material from an SM supply cartridge. A reflux safety valve 412connects reservoir 404 to a vacuum pump (not shown).

Operation of reservoirs 401 and 403 that supply BM to BM printing heads52 is optionally identical to operation of reservoirs 402 and 404 thatsupply SM to SM printing heads 52 and operation of the reservoirs willbe described with reference to reservoirs 401 and 403 and BM printingheads 52.

Referring to FIG. 2F, housing 56 of BM printing head 52 is formed with amanifold 420 that connects reservoir 401 and 403 and enables BM that thesupply pump pumps to reservoir 401 to flow freely into reservoir 403. Asensor (not shown) generates signals responsive to a height to which BMfills reservoirs 401 and 403. Supply pump control circuitry (not shown)controls operation of the supply pump to maintain a desired level of BMin reservoirs 401 and 403. FIG. 2F schematically shows reservoirs 401,403 and manifold 420 filled with BM indicated by shading 418.

A small feed line 422 formed from sections optionally having differentdiameters, as is known in the art, connects each output orifice 58 tomanifold 420 and is coupled to a piezoelectric actuator (not shown).Controller 26 (FIG. 1) controls the piezoelectric actuator coupled toeach feed line 420 to draw BM 418 from manifold 420 and expel desiredquantities of the BM from the feed line's associated output orifice 58.

To prevent unintentional dripping of BM from orifices 58 the vacuum pumpcoupled, optionally, to reservoir 403 maintains a slight vacuum inreservoirs 401 and 403. Reflux safety valve 411 prevents BM in reservoir403 from being accidentally drawn into the vacuum pump. Reflux safetyvalve 411 may function in accordance with any of various methods anddevices known in the art. Optionally, the reflux safety valve comprisesa float that rises to close a port in the valve through which the vacuumpump aspirates air, if and when BM enters the valve and rises above apredetermined level.

The inventors have found that a pressure in reservoirs 401 and 403between about 2 and about 9 mm H₂O below atmospheric pressure isadvantageous for preventing unintentional dripping of BM from orifices58. Monitoring of vacuum in reservoirs 401 and 403 and control of thevacuum pump that maintains the pressure may be accomplished using any ofvarious methods and devices known in the art. In some embodiments of theinvention, the vacuum pump operates continuously to draw air fromreservoir 403 and air flows slowly into reservoir 401 and/or 403 throughat least one vent. Suitable control circuitry controls the vacuum pumpto balance a rate at which the pump draws air from reservoir 403 and arate at which air flows into reservoir 401 and/or 403 through the atleast one vent and maintain the desired slight vacuum. In someembodiments of the invention, control circuitry controls the vacuum pumpto operate only when pressure in reservoir 403 rises above a desiredpressure.

As shuttle 28 moves along the x-axis dispensing construction material toprint a construction layer 34 (FIG. 1), droplets of constructionmaterial are dispensed from each orifice 58 of printing heads 52 asrequired onto construction platform 24 or onto a previously formed layer34 along a line, hereinafter a “deposition line”, parallel to thex-axis. Deposition lines for orifices 58 in a same printing head 52 thatdispense BM (i.e. orifices in a printing head 52 in a socket 54) areequally spaced one from the other by a distance equal to Δd_(y)(equation 1). A spatial resolution, hereinafter a “primary” y resolutionPR_(y), along the y-axis is therefore equal to Δd_(y) and constructionmaterial is optionally deposited in droplets comprising sufficientmaterial so that material deposited along adjacent deposition lines meldto form a smooth construction layer having substantially uniformthickness.

Deposition lines are schematically indicated by lines 70 in FIG. 3A forsome orifices 58 of printing heads 52 in sockets 54. Although thedeposition lines 70 are lines along a construction surface formed by RPA20, the deposition lines are shown projected onto the bottom of printinghead block 50 for convenience of presentation and to show theirrelationship to orifices 58 that determine their locations. FIG. 3Bshows a magnified portion of FIG. 3A in which deposition lines 70 andtheir relative locations are more easily seen than in FIG. 3A.

It is convenient to individualize printing heads 52 in sockets 54 withindexed labels P_(k), 1≦k≦4 and deposition lines 70 with indexed labelDL₁, DL₂ . . . DL_(M), which are shown in FIG. 3B (M is equal to thenumber of orifices 58 in a printing head P_(k) times the number ofprinting heads, i.e. optionally four). Every fourth deposition line 70is associated with an output orifice 58 in a same printing head P₁, P₂,P₃, or P₄ in a socket 54. For example, deposition lines DL₁, DL₅, DL₉, .. . are associated with printing head P₁.

Because of the distance between adjacent lines 59 of output orifices 58in printing block 50, as shuttle 28 moves, for example along thepositive x-axis, for locations at a same given x coordinate in aconstruction layer, construction material is dispensed at differenttimes by different printing heads. Let the speed with which shuttle 28moves along the x-direction be V_(S) and a distance between orificelines 59 in adjacent printing heads 52 be d_(X). Then a time delay“t_(d)” between times at which construction material is dispensed byadjacent printing heads 52 at locations in a construction layer having asame given x-coordinate is equal to about d_(X)/V_(S).

For example, if printing head P₁ deposits construction material at agiven x-coordinate along deposition lines DL₁, DL₅, DL₉ . . . at a timet₁, then printing head P₂ deposits construction material at the samegiven x-coordinate along deposition lines DL₂, DL₆, DL₁₀ . . . at timet₂, t_(d) seconds later. Relative times t₁, t₂, t₃ and t₄ at whichprinting heads P₁, P₂, P₃ and P₄ dispense construction material fromtheir output orifices 58 at a same given x-coordinate is represented byan extent to which their respective deposition lines extend to the rightin FIGS. 3A and 3B. The ends of deposition lines 70 and relative timest₁, t₂, t₃ and t₄ are indicated by lines labeled with the relative timesin FIG. 3B.

Droplets of liquid construction material that are deposited next to eachother have an affinity to each other and a tendency to coalesce. Thistendency to coalesce can generate imperfections in a construction layer,such as a construction layer 34 shown in FIG. 1, printed by RPA 20. Inparticular, the tendency to coalesce can result in a construction layerexhibiting striations parallel to deposition lines 70 along which RPA 20deposits construction material. Striations, when they appear, tend toappear in the neighborhoods of deposition lines 70 along which printinghead P₄ deposits construction material (i.e. DL₄, DL₈, DL₁₂, . . . ).

FIG. 4A is believed to illustrate a process by which striations areformed in a construction layer. The figure shows a sequence of schematictime-lapse, cross section views 81, 82, 83 and 84 through a constructionlayer along a plane parallel to the xz plane at a given x-coordinate.The time-lapse views illustrate deposition of droplets along depositionlines 70 by printing heads P₁-P₄ to form a construction layer.Time-lapse views 81, 82, 83 and 84 are assumed to be taken respectivelyat sequential times t₁, t₂, t₃ and t₄ respectively that are temporallyseparated by the transit delay time t_(d). Deposition lines along whichdroplets are deposited are indicated by circles labeled DL_(m). Dropletsof construction material that printing heads P₁-P₄ deposit are labeledDr₁-Dr₄ respectively.

At time t₁, in time-lapse view 81, printing head P₁ deposits dropletsDr₁ of material along deposition lines DL₁, DL₅, . . . at locationshaving the given x-coordinate. At time t₂, in time-lapse view 82,printing head P₂ deposits droplets Dr₂ at the given x-coordinate alongdeposition lines DL₂, DL₆, . . . . Each droplet Dr₂ is adjacent to apreviously deposited droplet Dr₁ and tends to coalesce with the dropletDr₁. At time t₃, in time-lapse view 83, printing head P₃ depositsdroplets Dr₃ adjacent to droplets Dr₂ along deposition lines DL₃, DL₈, .. . Droplets Dr₃ coalesce with the previously deposited droplets Dr₁ andDr₂ as shown in the time-lapse view.

It appears that material in the coalesced droplets does not readily flowinto empty regions 86 shown in time-lapse view 83, in the neighborhoodof deposition lines DL₄, DL₈, DL₁₂, . . . , between the coalesceddroplets. At time t₄, in time-lapse view 84, when printing head P₄deposits droplets Dr₄ into empty regions 86, material in each droplet isdrawn away to each of the groups of previously coalesced droplets oneither side of the droplet. The drawing away of the material generates aslight lacuna 88 in the neighborhood of deposition line DL₄, as shown intime-lapse-view 84. Lacunae 88 give rise to striations in constructionlayers formed by RPA 20.

To obviate striations, in accordance with an embodiment of the presentinvention, y alignment buttons 66 comprised in sockets 54 are configuredso that each droplet dispensed at a given x-coordinate, followingdeposition of material by a first printing head at the x-coordinate, isdeposited equidistant between previously deposited droplets. Theinventors have determined that when an “equidistant” method of dropletdeposition is used to form a construction layer, striations that mightoccur in the construction layer were the droplets deposited asillustrated in FIG. 4A, are moderated or are substantially non-existent.It is noted that equidistant deposition can be exactly and completelyimplemented for deposition of droplets of construction material in alayer only if a number of deposition lines used to construct the layeris equal to a power of two. Otherwise, the method can be implementedonly approximately.

FIG. 4B schematically illustrates “equidistant” deposition ofconstruction material to form a construction layer of an object, inaccordance with an embodiment of the invention. The figure is similar toFIG. 4A and shows a sequence of schematic time-lapse cross section views91, 92, 93 and 94. The cross section views are along a plane parallel tothe xz plane at a given x-coordinate and illustrate deposition ofdroplets of construction material deposited at sequential times t₁, t₂,t₃ and t₄ along deposition lines in accordance with equidistantdeposition.

At time t₁, in time-lapse view 91, droplets Dr₁ are deposited alongdeposition lines DL₁, DL₅, DL₉ . . . . At time t₂, in time-lapse view92, droplets Dr₂ are deposited on deposition lines DL₂, DL₆ . . . notadjacent to droplets Dr₁ but equidistant between the droplets alongdeposition lines DL₃, DL₅ . . . . At time t₃, in time-lapse view 93,droplets Dr₃ are optionally deposited along deposition lines DL₂, DL₄ .. . . From times t₁ and t₂ to time t₃, material in droplets Dr₁ and Dr₂respectively, spread. Spreading of droplets Dr₁ and Dr₂ is believed topartially fill regions 96 along deposition lines DL₄, DL₈, DL₁₂. As aresult of the filling, when, in time-lapse view 94, droplets Dr₄ aredeposited at time t₄ along deposition lines DL₄, DL₈, DL₁₂, lacunae arenot formed along the deposition lines and striations are not formed.

FIG. 4C schematically shows a bottom view of printing head block 50configured to implement equidistant deposition illustrated in FIG. 4B.The lengths of y alignment buttons 66 in sockets 54 (and optionallysockets 53) do not decrease linearly with increase in their x-coordinaterelative to the x-coordinate of a feature of printing head block 50. Asa result, printing heads P₁, P₂, P₃, P₄ do not deposit material alongdeposition lines DL₁, DL₂, DL₃ and DL₄ respectively as shown in FIGS. 3Band 4A. Instead, they are configured so that printing heads P₁, P₂, P₃,P₄ deposit material along deposition lines DL₁, DL₃, DL₂ and DL₄respectively, as shown in FIG. 4B.

Whereas the alignment features comprised in printing head block 50 andprinting heads 52 enable replacement of a printing head 52 in theprinting head block without having to adjust or calibrate alignment ofthe printing head, a given printing head will, in general, becharacterized by operating characteristics that are peculiar to theprinting head. To provide for proper operation of a given printing head52, it is advantageous for controller 26 to control each printing headresponsive to its peculiar operating characteristics. In accordance withan embodiment of the invention, each printing head 52 is profiled byprofile data that characterizes operating parameters peculiar to theprinting head. Optionally, as noted above profile data is stored in amemory 49, optionally comprised in the printing head's circuit board 55.When printing head 52 is mounted in a socket 51 data lines betweencontroller 26 and printing head 52 over which the controller accessesthe printing head's profile data are established via connectors 47comprised in the circuit board.

Profile data that characterizes a printing head 52 may, for example,comprise operating data that specifies operation of each piezoelectricactuator comprised in the printing head that controls deposition ofconstruction material via an output orifice 58 of the printing head.Typically, operating data for the actuator specifies actuatorperformance as a function of voltage applied to the actuator, identityand temperature of the construction material that printing head 52dispenses. The data is generally used to determine rise time, fall timeand amplitude of a voltage pulse that controller 26 applies to theactuator to control weight and/or ejection velocity of a drop ofconstruction material dispensed through an orifice 58 with which theactuator communicates. Profile data optionally comprises operatingcharacteristics of a heater optionally comprised in printing head 52,which heater controller 26 controls to maintain a desired temperature ofconstruction material in the printing head reservoir.

Profile data may also comprise dimensional data for a printing head. Forexample, in some embodiments of the invention, lengths of x alignmentpins 60 (FIG. 2D), while controlled so that the x pins on a sameprinting head 52 are a same length Δx to a high degree of accuracy, mayvary by relatively large amounts from one printing head 52 to another.As a result, an a priori length of x pins 60 may not be known a priorifor each printing head 52 to a degree of accuracy required for a desiredresolution of RPA 20. For such embodiments, profile data for a printinghead comprises data defining the lengths of its x alignment pins.

Whereas in the exemplary embodiment discussed above, optionally a memory49 located on a printing head's circuit board 55 (FIG. 2D) comprisesprofile data for the printing head, in some embodiments of the inventionprofile data for a printing head 52 is comprised in a memory deviceseparate from the printing head. For example, optionally a floppy disk,CD or portable flash memory comprises profile data for a printing head52. The data is downloaded from the memory device to controller 26 usingany of various methods and devices known in the art when the printinghead is inserted into a socket 51 of printing head block 50 (FIGS.2A-2C).

In the above-described exemplary embodiment, printing heads 52 areinserted into individual sockets 51 in printing block head 50. In someembodiments of the invention a printing head block does not havesockets. FIG. 5 schematically shows a bottom view of a printing headblock 100 that does not comprise individual sockets for each printinghead mounted to the block, but instead comprises a single mountingcavity 102 for receiving printing heads 104.

Printing heads 104 are optionally identical and each is fitted with twox alignment pins 60 and a y alignment pin 62. In addition, each printinghead 104 is fitted with two x alignment buttons 106. Mounting cavity 102comprises y alignment buttons 108 and associated resilient elements 110that correspond to y alignment pins 62 comprised in the printing heads104 for, by way of example, eight printing heads 104. Lengths of yalignment buttons optionally increase linearly with increase in theirx-coordinate relative to the x-coordinate of a feature in printing headblock 100. Mounting cavity 102 also comprises two x alignment buttons112 and corresponding resilient elements 114.

When eight printing heads 104 are inserted into mounting cavity 102,resilient elements 114 urge the printing heads one to the other alongthe x direction. As a result, x pins 60 of one printing head are pressedto x buttons 106 of a next printing head and the x pins of a lastprinting head press on x alignment buttons 114 in the cavity. Resilientelements 110 urge printing heads 104 so that their y pin press against ybuttons in mounting cavity 102. The operation of the x and y alignmentpins in printing heads 104 and corresponding x and y buttons andresilient elements in mounting cavity 102 operate to align the printingheads.

Each radiation lamp 120 (as shown for example in FIG. 2A) comprised inshuttle 28 optionally comprises a UV light bulb 122 that provides UVlight for polymerizing construction material dispensed by printing heads52, a reflector 124 and a housing 126 that supports and contains thereflector and bulb. UV light bulb 122 is optionally a discharge typebulb such as a Mercury or Xenon discharge bulb. Optionally, lamp 120comprises a protective cover plate 128 that is transparent to UV lightprovided by bulb 122 and covers an aperture 129 of the lamp throughwhich it provides light.

FIG. 6A schematically shows an enlarged view of shuttle 28, shown inFIG. 1, in which components of a lamp 120, in accordance with anembodiment of the invention, are shown as seen through housing 126 ofthe lamp, whose outline is indicated by dashed lines. In the figurereflector 124 is shown partially cutaway. FIGS. 6B and 6C show crosssectional views of lamp 120 in planes indicated by lines AA and BB.

UV light provided by lamp 120 that is reflected back to the printingheads 52 from a construction layer formed by RPA 20 and or surfaces ofconstruction platform 24 (FIG. 1) may polymerize construction materialon a printing head 52 (FIG. 2A) or other parts of shuttle 28.Polymerized construction material on a printing head 52 may block anoutput orifice 58 or orifices on the head. In addition, clumps ofhardened or partially hardened construction material on a printing head52 or other region of shuttle 28 may fall onto or collide with anobject, such as object 22 (FIG. 1), that is being built by the RPA anddamage the object.

The inventors have found that an amount of light reflected back from aconstruction layer to shuttle 28 is a function of a height above theconstruction layer and surface regions of construction platform 24 atwhich the lamp provides the light. The form of dependence of the amountof reflected light that reaches shuttle 28 as a function of height issimilar to that shown in FIG. 6D in a graph 180, which graphs the amountof reflected light “RR” reaching the shuttle as a function of the height“H”.

Whereas, the amount of reflected light is relatively small forrelatively large as well as for relatively small values of H, it is ofcourse advantageous to make H relatively small rather than relativelylarge in order to use light provided by lamp 120 efficiently. Therefore,in accordance with an embodiment of the invention lamps 120 are mountedto shuttle 28 so that in general during printing of construction layersby RPA 20 their respective apertures 129 are relatively close to theconstruction layers. In some embodiments of the invention, apertures 129are less than about 10 mm from construction layers produced by RPA 20.In some embodiments of the invention apertures 129 are less than about15 mm from construction layers produced by RPA 20. In some embodimentsof the invention apertures 129 are less than about 10 mm fromconstruction layers produced by RPA 20. In some embodiments of theinvention apertures 129 are about 5 mm from construction layers producedby RPA 20.

A problem often encountered in the production of objects by a jet-inkRPA, such as RPA 20, is that it can be relatively difficult to providethe objects with sharply defined edges and features. Material alongedges of a construction layer of an object produced by an RPA tends to“run” during production and, as a result, the edges tend to deform andlose definition. The inventors have determined that material along edgesurfaces of a construction layer of an object tends to be relativelyslowly and inefficiently polymerized and that this relatively slow andinefficient polymerization contributes to the poor definition of edgesand fine detail in an object. In addition inefficient polymerization mayalso leave edges in the object unhardened and “sticky”.

Inefficient, slow or partial polymerization of material along edgesurfaces evidenced in an object produced by a prior art RPA appears toresult from polymerizing light provided by lamps in the prior art RPAhaving relatively low intensity and being relatively strongly reflectedfrom edge surfaces.

Increasing intensity of polymerizing light provided by a UV lamp doesnot in general alleviate the problem. Most of the material in the bodyof a construction layer of an object formed by an RPA is relativelyrapidly polymerized at UV light intensities that are not sufficient torapidly and effectively polymerize construction material along edgesurfaces of the layer. Increasing intensity of the UV light is thereforewasteful of energy and most of the increase in intensity goes intoheating material in the body of the layer that is already polymerized.The increased heating increases heat stress in components of the RPA andin the layer, tends to generate distortions in the layers and degradesaccuracy with which the object is formed and quality of the object.

In accordance with an embodiment of the invention, to increaseefficiency with which a UV lamp provides light that polymerizesconstruction material along edges of a construction layer without undulywasting energy in undesired heating, the lamp provides light atrelatively large angles to the normal to the plane of the constructionlayer. For a given intensity of light provided by the lamp, a ratio ofintensity of light incident on edge surfaces of the layer to thatincident on surfaces parallel to the plane of the layer increases as theangle of incidence increases. As a result, efficiency of polymerizationof construction material along the edges increases relative to that ofmaterial in the body of the layer as the angle of incidence increases. Asuitable angle of incidence and intensity of UV light can therefore bedetermined, in accordance with an embodiment of the invention, so thatthe light effectively polymerizes material in the edges as well as inthe body of a construction layer without inordinate heating and waste ofenergy. Material in edges of construction layers produced by an RPAhaving a UV lamp in accordance with an embodiment of the invention isrelatively efficiently polymerized. As a result the edges are not assusceptible to running and deformation as are edges of constructionlayers produced by prior art RPAs and tend to have improved definition.

By way of example, UV lamps 120 comprised in shuttle 28 provide a largepart of their UV light output at angles of incidence equal, optionally,to about 45°. Optionally, reflector 124 in the UV lamps comprises anedge reflector 130 and optionally, planar reflectors 132, which are,optionally, surfaces of housing 126 that are treated so that theyreflect light provided by bulb 122. Optionally, edge reflector 130comprises two mirror image parabolic reflectors 134 that meet along acommon edge 136 and are positioned so that their respective focal spotsare substantially coincident. Radiation bulb 122 is optionally mountedto edge reflector 130 through suitable holes in the reflector. Contactends 138 of bulb 122 are mounted to power sockets (not shown) comprisedin housing 126 that provide electrical contact of bulb 122 to a powersupply (not shown). Optionally, the sockets provide support for bulb 122and maintain the bulb in position in housing 126.

Bulb 122 has a localized “hot spot” 140 from which most of the lightprovided by the bulb emanates and is positioned so that hot spot 140 islocated substantially at the focal spots of parabolic reflectors 134.Each parabolic reflector 134 is positioned so that a relatively largeportion of light that emanates from hot spot 140 is reflectedsubstantially at an angle of about 45° to cover plate 128 through whichthe light exits lamp 120 and is incident on a construction layer beingformed by RPA 20.

The cross sectional view of lamp 120 in FIG. 6B schematically showsparabolic reflectors 134 reflecting rays 150 of UV light from hot spot140 so that the light exits the lamp through cover plate 128 at about450 to the plane of the cover plate. The reflected light is incident ona region of a construction layer 152 produced by RPA 20. Layer 152 hasedges 154 that are shown greatly magnified in insets 156. UV light thatexits lamp 120 at about 450 to the plane of construction layer 152, inaccordance with an embodiment of the invention, is incident on surfaceregions of edges 154 along directions that are relatively close to thedirections of normals, indicated by block arrows 158, to the edgesurfaces. As a result, relative intensity of light incident on surfacesof edges 154 is increased and a relatively large portion of the incidentlight penetrates into construction material along the edges and iseffective in polymerizing the material.

FIG. 6C schematically shows planar reflectors 132 reflecting rays oflight 159 from hot spot 140 so that they exit cover plate 128. Toprovide relatively intense light to polymerize material in aconstruction layer formed by RPA 20, optionally, planar mirrors arerelatively close to each other so that light provided by bulb 122 thatexits lamp 120 is concentrated on a relatively small surface region ofthe construction layer. The inventors have determined that therelatively close planar reflectors contribute to reducing an amount ofUV light provided by lamp 120 that is reflected towards orifices inprinting heads comprised in shuttle 28.

FIGS. 7A and 7B schematically show cross sectional views of variationsof UV lamp 120. The cross sectional views are in the plane indicated byline AA shown in FIG. 6A and are similar to that shown in FIG. 6B. InFIG. 7A an edge reflector 160 in accordance with an embodiment of theinvention and similar to edge reflector 124, comprises four parabolicreflectors 161, 162, 163 and 164. Parabolic reflectors 161 and 163 aremirror images of each other and parabolic reflectors 162 and 164 aremirror images of each other. Focal spots of all parabolic mirrorssubstantially coincide with hot spot 140 of bulb 122. In FIG. 7B an edgereflector 170, in accordance with an embodiment of the invention,similar to edge reflector 124, comprises two “prismatic” parabolicreflectors 171 and 172 and planar reflectors 173 and 174. Parabolicreflectors 171 and 172 are mirror images of each other and eachcomprises two planar panels 175. Planar reflectors 172 and 174 aremirror images of each other.

Discharge type bulbs, such as Hg and Xe discharge bulbs, that areconventionally used to provide UV light, generally require a highvoltage power supply and cumbersome ignition system for their operation,generate relatively large amounts of heat and cannot be turned on andoff rapidly.

In some embodiments of the invention UV lamps comprise LEDs that provideUV light for polymerizing construction material. UV LEDs generaterelatively small amounts of thermal energy in comparison with the UVenergy they deliver, can be turned on and off relatively rapidly and canprovide UV radiation in a relatively small bandwidth of desiredradiation. Output intensities of LEDs can be relatively easilycontrolled and they can be packaged in arrays sufficiently dense toprovide UV light at intensities required for rapid polymerization ofconstruction materials used by RPAs.

Turning the LEDs on and off is an immediate operation, not involvingtime delays or RF (radio frequency) interference radiation typical ofoperation of discharge type bulbs. The process of building an objectstarts quickly and the process itself is more reliable due to theaforesaid immediate on/off switching of the LEDs.

Furthermore, use of LEDs would decrease deformation of the printed modelfor a number of reasons, for example, a significant difference intemperature between the object (during the building process) and roomtemperature is a cause of deformation in the final printed object aftercooling, especially when cooling is carried out fast and not evenlythroughout the process. As LED arrays dissipate only a small amount ofheat per curing quantity, the built object is processed in lowertemperature conditions than when discharge lamps are used and thus thedeformation liable to occur during cooling of the object is lessened.

FIG. 8 schematically shows a shuttle 28 comprising UV lamps 190, each ofwhich optionally comprises an optionally densely packed array 191 ofLEDs 192 that provide UV light. Optionally, LEDs 192 are in DIE form(i.e. semiconductor dice, and in this case non-packaged LEDs) and arearrayed at a pitch of about 1 mm. Optionally, LEDS 192 are SMD LEDs,which may be configured in array 191 at a pitch less than 2 mm. Toprovide UV radiation that is incident at relatively large angle ofincidence on regions of a construction layer formed by RPA 20,optionally, each LED 192 is coupled to a microlens using methods knownin the art that shapes light provided by the LED into substantially acone beam of light having a relatively wide cone angle. Optionally, thecone angle is larger than about 80° (full cone angle). Optionally, thecone angle is larger than about 100° (full cone angle). Optionally,controller 26 controls intensity of light provided by a UV LED 192 bycontrolling current or voltage supplied to the LED. Optionally,controller 26 delivers power to a LED 192 in the form of a train ofcurrent or voltage pulses and the controller controls a duty cycle ofthe pulse to control intensity of UV light from the LED.

In accordance with an embodiment of the invention, controller 26(FIG. 1) that controls operation of shuttle 28 controls intensities ofUV light provided by LEDs 192 in array 191 independently of intensitiesprovided by other LEDs in the array. In particular, the controllercontrols individual LEDs 192 so as to limit UV radiation that lamp 190provides to where and when it is needed. For example, as a constructionlayer 34 is printed, the layer may have non-printed regions whereconstruction material is not deposited. Optionally, controller 26controls LEDs 192 so that the non-printed regions receive relativelylittle or substantially no UV light. During production of an object,such as object 22, as noted above, controller 26 periodically initiatesa maintenance procedure and moves shuttle 28 away from constructionplatform 24 to maintenance areas 200 for cleaning. For duration of themaintenance procedure, controller 26 optionally shuts off LEDs 192.

Whereas LEDs 192 generate relatively little heat, they and/or circuitryassociated with the LEDs do generate heat, and in a densely packedarray, it can be advantageous to provide lamps 190 with features toenhance heat dissipation. In some embodiments of the invention, LEDs 192are mounted to appropriate heat sinks and/or coupled to Peltier devices,and/or are provided with suitable fans for enhancing heat dissipation.

In some embodiments of the invention, an RPA similar to RPA 20, inaccordance with an embodiment of the invention comprises a shuttle inwhich LEDs are positioned relatively far from construction layers thatthe RPA produces. UV light form the LEDs are piped to the constructionlayers by light pipes or optical fibers.

FIGS. 9A and 9B schematically show perspective views of a shuttle 194comprising LEDs 196 that are positioned relatively far from constructionlayers that the shuttle prints. FIG. 9A shows a perspective view ofshuttle 194 from the bottom. FIG. 9B shows a perspective view of theshuttle “right side up” and a construction layer 198. LEDs 196 arecoupled to optic fibers or light pipes 200 that pipe light from the LEDsto the construction layer. UV light from LEDs 196 exit light pipes 200via ends 202, which are supported by a suitable support structure orhousing (not shown) in close proximity to construction layer 198 (FIG.9B). Optionally, ends 202 are coupled to or formed with a suitable lensso that UV light exits in a cone of light having a relatively large coneangle. LEDs 196 and optionally circuitry associated with the LEDs aresupported or mounted in a housing (not shown) in a relatively “open”configuration to enhance heat dissipation.

As noted above, periodically during production of an object, controller26 moves shuttle 28 to maintenance area 220 (FIG. 1) and performs acleaning procedure. The cleaning procedure generally comprise a purgingprocedure in which construction material is released form all orifice atone to refresh the flow of material through the printing head.Controller 26 then controls. shuttle 28 to contact an edge of at leastone of first cleaning blade 225 and second cleaning blade 227 and movein a direction substantially perpendicular to the edge so that thecleaning blade wipes away residual droplets of material remaining on theorifice surface after purging as well as excess construction material“debris” and dirt that accumulates on surfaces of printing heads 52during production.

FIGS. 10A and 10B schematically show an enlarged perspective view andcross section view respectively of the bottom of shuttle 28 during acleaning procedure in accordance with an embodiment of the invention.The figure shows cleaning blades 225 and 227 removing constructionmaterial debris 229 from and wiping clean, surfaces, hereinafter“orifice surfaces” 230, of printing heads 52 in which output orifices 58are located.

Cleaning blades 225 and 227 have “cleaning” edges 226 and 228respectively that are optionally parallel to each other and to the xaxis. Optionally, cleaning edge 226 of first cleaning blade 225 is closeto but displaced from and does not contact orifice surfaces 230.Cleaning edge 228 of second cleaning blade 227 contacts orifice surfaces230. Shuttle 28 moves parallel to the y-axis in a direction indicated bya block arrow 232. As shuttle 28 moves, edge 226 of first cleaning blade225 removes relatively large accumulations of debris that protrudesubstantially from orifice surfaces 232. Edge 228 of second cleaningblade 227 removes remaining debris and scrapes the surfaces clean.

Debris 229 removed from the surfaces of printing heads 52 by cleaningblades 225 and 227 falls or drips into sump 222 shown in dashed lines. Asufficient distance separates first and second wiping blades 225 and 227so that debris removed from orifice surfaces 230 by cleaning edges 226and 228 of the blades is not hindered from dripping or falling into sump222. A suitable vacuum pump (not shown) removes debris accumulated insump 222 during maintenance procedures.

The inventors have found that by using two cleaning blades a pre-wiper,i.e. first cleaning blade 225, that does not quite contact surfaces 230of printing heads 52 and a scraper, i.e. second cleaning blade 227, thatcontacts and scrapes the surfaces, a tendency of debris to accumulatebetween the printing heads during cleaning is reduced.

In some embodiments of the invention, edges of cleaning blades are notstraight but have a crenulated or scalloped shape. FIG. 10Cschematically shows a cleaning blade 240 having a scalloped a edge 242.A straight cleaning blade edge tends to push portions of debris that theblade scrapes from printing heads 52 laterally along the blade edge.Debris that is forced along the edge has a tendency to get caught andaccumulate in spaces between the printing heads. A scalloped edge tendsto prevent lateral movement of removed debris and direct the debrisdownward to sump 222.

A scalloped edge is not the only shaped edge that functions to preventlateral movement of debris along the edge. FIG. 10D schematically showsa cleaning blade 244 having an edge 246 shaped like a train oftriangular pulses, which for example, will perform similarly.

In some embodiments of the invention, cleaning area 220 (FIG. 1)comprises a single cleaning blade. FIGS. 11A and 11B schematically showperspective and cross section views respectively of shuttle 28undergoing maintenance cleaning during which, optionally, a singlecleaning blade 248 cleans orifice surfaces 230 of printing heads 52.

Cleaning blade 248 comprises a thin elastic blade optionally formed fromplastic, rubber or metal. Optionally, cleaning blade 248 is formed froma thin sheet of steel about 50 microns thick. Cleaning blade 248 ismounted over sump 222 so that it is angled with respect to surfaces 230of printing heads 52. During cleaning, controller 26 (FIG. 1) positionsshuttle 28 so that surfaces 230 (FIG. 11B) press down on cleaning blade248 causing the blade to contact the surfaces at an acute angle and acleaning edge 250 of the blade to press resiliently to the surfaces. Asshuttle 28 moves in the direction of block arrow 232, cleaning edge 250efficiently scrapes debris 229 off surfaces 230 so that it drips and/orfalls into sump 222.

In some embodiments of the invention, a cleaning blade similar tocleaning blade 248, in accordance with an embodiment of the invention,is slotted so that it comprises a plurality of individually flexibleteeth. FIG. 11C schematically shows a slotted cleaning blade 260, inaccordance with an embodiment of the invention, cleaning shuttle 28.Cleaning blade 260 comprises a plurality of teeth 262 having cleaningedges 264. During cleaning each tooth 262 contacts a surface 230 of adifferent printing head 52 at an acute angle and an edge 264 of thetooth presses resiliently to the surface. Since each tooth 262 isflexible substantially independently of the other teeth, each tooth 262adjusts to the height, i.e. the z-coordinate, of surface 230 of theprinting head 52 that it cleans independently of the other teeth.Cleaning blade 260 is therefore able to compensate efficiently to slightdifferences in the heights of surfaces 230.

It is noted that slotting, in accordance with an embodiment of theinvention, is not advantageous only for blades that function like blades248 and 260. Cleaning blades similar to blades 225 and 227 (FIG. 10A)and blades 242 and 246 may also be slotted so that in effect each bladecomprises a plurality of small cleaning blades (i.e. teeth), each ofwhich cleans a different printing head 52 and adjusts substantiallyindependently to differences in heights of surfaces 230 of the heads.

Despite implementation of regular maintenance cleaning of printing heads52, during construction of an object, construction material debris mayfall on a construction layer, or during leveling of a constructionlayer, the layer may be damaged, leaving it, in either case withunwanted protuberances. For such situations, not only may protuberancesin the layer damage quality of a next layer to be deposited on thedamaged layer, but as shuttle 28 moves over the construction layer itmay collide with the protuberance and be damaged.

Therefore, an RPA, in accordance with an embodiment of the invention,such as RPA 20, optionally comprises an obstacle detection system. Thedetection system generates signals responsive to unwanted protuberancesthat may be formed on a construction layer and transmits the signals tocontroller 26. The controller either undertakes corrective action, suchas attempting to level the layer using leveling roller 27, or stopsproduction of the object and generates an alarm indicating that userintervention is required.

FIGS. 12A and 12B schematically show a perspective view and a crosssection view respectively of an RPA 300 similar to RPA 20 and comprisingan obstacle detection system 302, in accordance with an embodiment ofthe invention. Only components and features of RPA 300 germane to thediscussion are shown in FIGS. 12A and 12B. In the figures, RPA 300 isshown forming layers 304 of construction material during production ofan object (not shown) and detecting protuberances in a top constructionlayer 306.

Obstacle detection system 302 optionally comprises a laser 308 andassociated optics as required (not shown), controllable by controller 26to provide, optionally, a pencil beam 310 of laser light. The detectionsystem comprises an optical detector 312, and associated optics asrequired (not shown), for detecting light provided by laser 308.Optionally, laser 308 and detector 312 are mounted to carriages 314 and315 respectively that sit in slots 316 and 317 formed in working table25. Carriages 314 and 315 are optionally mounted to threaded shafts 318and 319 located in slots 316 and 317 respectively. The slots areoptionally parallel to the y-axis. Controller 26 controls at least onemotor (not shown) to rotate shafts 318 and 319 and position carriages314 and 315 at desired locations along their respective slots 316 and317 and thereby at desired y-coordinates. Optionally, laser 308 anddetector 312 are controllable by controller 26 to be raised and loweredin directions perpendicular to worktable 25 (i.e. parallel the z-axis).

To detect protuberances in top construction layer 306, controller 26positions laser beam 10 so that it contacts the surface of the layeralong a length of the laser beam and moves carriage 314 along slot 316so that, as it moves, protuberances that may be present in the layer atleast partially block light in the laser beam. As controller 26 moveslaser 308 it moves detector 312 to detect light from the pencil beam 10.Signals generated by detector 312 responsive to light in beam 10indicate if and when the beam is blocked and thereby presence of aprotuberance. FIG. 12C schematically shows laser beam being blocked by aprotuberance 320. Optionally, controller 26 moves laser 308 and detector312 so that pencil beam 10 precedes shuttle 28 as it moves along they-axis and “scans” a region of top layer 306 for protuberances justbefore printing heads in the shuttle overprint the region withconstruction material for a next construction layer.

It is noted that for the configuration of obstacle detection system 302shown in FIG. 12A and 12B, motion of laser 308 and detector 312 arelimited along the z-axis. The limitation does not affect ability ofdetection system 302 to detect protuberances in a top construction layerbecause it has been assumed for RPA 300, as for RPA 20, that for eachnew construction layer, construction platform 24 is lowered bysubstantially a layer thickness. As a result, all construction layersproduced by RPA 300 are produced at substantially a same height aboveworktable 25, i.e. at a same z-coordinate, or at heights above theworktable within a same small range of heights.

However, in some RPAs in accordance with embodiments of the inventionconstruction layers are not all produced at a substantially samez-coordinate. Instead the RPA's shuttle is raised by a layer thicknessfor each construction layer of at least some new layers that the RPAproduces. For such embodiments, it can be advantageous, if notnecessary, for an obstacle detection system to have a dynamic rangealong the z-axis substantially larger than that of detection system 302.An obstacle detection system, in accordance with an embodiment of theinvention, can of course, where required or advantageous, be provided sothat it has a substantially larger dynamic range along the z-axis thanthat of detection system 302.

FIG. 12D schematically shows an obstacle detection system 330 inaccordance with an embodiment of the invention, which is a variation ofsystem 300. Obstacle detection system 330 has a dynamic range along thez-axis substantially larger than that of system 302. Detection system330 optionally comprises carriages 332 each having a slider 334controllable to be raised and lowered. A laser 308 and detector 312 aremounted to sliders 334 in different carriages 332 and are optionallycontrollable to be positioned at different locations along slider'slength in the z-direction. A dynamic range for positioning laser 308 anddetector 312 is substantially equal to a dynamic range of motion ofsliders 334 in the z-direction plus substantially an extent of thesliders in the z-direction. Alternatively, by way of another example, alaser and detector for detecting protuberances may be mounted to shuttle28 so that they move parallel to the z-axis with the shuttle.

It is noted that obstacle detection systems in accordance withembodiments of the invention, such as for example detection systems 302and 330, can be used not only to detect protuberances in constructionlayers but also the presence of obstacles on construction platform 24.Such obstacles may, for example, comprise pieces of a first objectconstructed by an RPA and inadvertently left on the RPA's constructionplatform that might interfere with production of a second, subsequentobject by the RPA.

Optionally, in accordance with an embodiment of the invention, an RPAcomprises a collision detection system for detecting if and when theRPA's shuttle collides with an obstacle. Upon occurrence of a collision,controller 26 optionally stops production of an object and generates analarm to alert a user that a collision has occurred and that his or herintervention is require.

By way of example, RPA 300 shown in FIGS. 12A-12D is shown comprising acollision detection system 340 optionally mounted on shuttle 28.Optionally, collision detection system 340 comprises an accelerometer(not shown) that generates signals responsive to acceleration of shuttle28. A collision usually generates a force that produces an unwantedshuttle acceleration having a characteristic profile useable to identifythe acceleration as resulting from a collision. For example, a collisionin general results in an impulse applied to the shuttle that produces acorresponding identifiable acceleration.

Whereas in FIGS. 12A-12D collision system 340 is shown mounted onshuttle 28, a collision detection system, in accordance with anembodiment of the invention, may be mounted in or on other components ofan RPA. For example, a collision detection system may be mounted on acomponent (not the shuttle) of the RPA to detect vibrations in thecomponent characteristic of those generated by a collision. In someembodiments of the invention, a collision detection system comprises amicrophone and associated algorithms for identifying sounds thattypically accompany a collision.

Printing resolution of droplets of construction layers dispensed by anRPA (i.e. density of droplets of construction material printed along thex and y directions) and other “operating” parameters that defineoperating specifications of an RPA are generally complex functions ofeach other. For example, “waste ratio” is conventionally defined as aratio of quantity of construction material removed from a printedconstruction layer by leveling roller 27 (FIG. 1) to an amount ofmaterial printed to form the layer. With increase in waste ratio,generally, thickness of a layer decreases, quality of constructionincreases (resolution of construction in the stacking direction, i.e.z-direction, increases), production speed decreases and cost increases.To provide effective operation of an RPA, values for its operatingparameters are determined responsive to their interdependencies. Since,interdependencies of an RPA's operating parameters are generallymanifold, it is usually complicated to determine a set of values for theoperating parameters that provide for efficient operation of the RPA.Resolution and other operating parameters of an RPA are thereforeusually factory set and are not adjustable by a user. As a result, auser has limited flexibility in determining production specifications,hereinafter “object specifications”, which define desired qualities andcharacteristics of an object that the RPA produces.

To provide flexibility and expanded user control of operating parametersof an RPA and thereby of object specifications, in accordance with anembodiment of the invention, the RPA's controller is provided with “RPA”operating algorithms and data. The RPA operating algorithms and dataenable a user to adjust an RPA's operating parameters responsive todesired specifications for an object that the RPA produces. The userinputs information to the RPA that defines desired object specificationsand the controller adjusts operating parameters of the RPA responsive tothe RPA data and algorithms to satisfy the object specifications. If aparticular profile of object specifications cannot be met, thecontroller communicates to the user that they cannot be met andinstructs him or her as to which object specification options areavailable and how to proceed to set an acceptable object specificationprofile.

For example, controller 26 of RPA 20 is optionally provided with datathat correlates values for layer thickness (LT), x and/or y-axisprinting resolution (PR), and driving voltage (DV) for printing heads 52that controls volume of droplets of construction material that theprinting heads dispense. FIG. 13 shows a schematic graph 350 of RPA dataavailable to controller 26. Graph 350 shows a surface 360 that relatesvalues of layer thickness LT, resolution PR and operating voltage DV fora printing head 52. (Operating data such as that represented by graph350 may be different for different printing heads. As noted above, inaccordance with an embodiment of the invention, such operating data isprovided to controller 26 for each printing head 52 by memory 49comprised in the printing head circuit board 55.) Resolution is assumedmeasured in units of dpi, dots or droplets of construction materialdispensed per inch along the x or y-axis shown, e.g., in FIG. 1, toproduce a construction layer. Lines 361 and 362 on surface 360 aid invisualizing the surface and lie in planes parallel respectively to theDV-LT and PR-LY planes in graph 350. Lines 361 and other lines in planesparallel to the DV-LT plane show layer thickness LT as function of drivevoltage DV for different constant values of resolution PR. Lines 362 andother lines in surface 360 that are parallel to the PR-LT plane show LTas function of PR for different constant values of DV. When a userspecifies a desired construction layer LT and a printing resolution PR,controller 26 determines driving voltage in accordance with anappropriate RPA algorithm responsive to the data represented in graph350. If there is no driving voltage that provides the specified LT andPR, controller 26 alerts the user to that fact and presents the userwith acceptable ranges for LT and RP.

From graph 350 it is seen that for a given driving voltage DV, layerthickness LT of a construction layer in an object produced by an RPAincreases as printing resolution PR increases. This is because for agiven DV, droplets of construction material dispensed by a printing head52 comprise substantially a same volume of material and as resolutionincreases the density of droplets deposited per inch, i.e. dpi, alongthe x and/or y-axis increases. As a result, more material is depositedper unit surface area of the construction layer and the thickness of thelayer increase. However, as thickness of construction layers increase,construction resolution along the z-axis decreases and fidelity of theobject produced by the RPA to an article of which the .object is a copyis reduced. The effects of surface tension of construction materialprinted to form a construction layer also reduce fidelity and quality ofthe produced object Surface tension of the-construction material tendsto deform edges of a construction layer and deformation of edges tendsto increase with increase in layer thickness.

In accordance with an embodiment of the invention, an object constructedby an RPA that has fidelity and quality provided by relatively highprinting resolution along the x and/or y-axis is produced fromrelatively thin construction layers.

Let x-pitch and y-pitch of a construction layer in the object be thedistances between coordinates at which construction material dropletsare deposited along the x and y-axes respectively to form the layer.(The x-pitch and y-pitch are the inverses respectively of the x and yprinting resolutions.) Layers in the object are printed at relativelylow x and/or y printing resolutions (not necessarily the same) andcorresponding relatively large x-pitch and/or y-pitch so that the layersare relatively thin and are not as sensitive to surface tension effectsas thick layers. However, in accordance with an embodiment of theinvention, the x and/or y-coordinates at which droplets of constructionmaterial are deposited in adjacent construction layers are shifted fromeach other by a fraction, a “pitch fraction”, less than one of thex-pitch and or y-pitch of the droplets. (The pitch fraction in notnecessarily the same for both x and y coordinates.)

The inventors have found that fidelity and quality of the object aresubstantially that of an object constructed from relatively thin layershaving “effective” x and/or y resolutions equal to the relatively lowresolutions at which the layers are actually printed multiplied by theinverse of the corresponding pitch fraction. Since the pitch fraction isless than one, the effective resolutions are substantially increased.

FIG. 14 schematically illustrates printing layers in accordance with anembodiment of the invention as described above. The figure shows aschematic cross section, by way of example parallel to the xz plane, ofconstruction layers 381, 382, 383 formed by an RPA, in accordance withan embodiment of the invention. Layers 381, 382 and 383 are formed fromdroplets of construction material 391, 392, and 393 respectively. Thedroplets are schematically shown after they have melded and been leveledby a leveling roller, such as leveling roller 27 (FIG. 1). The printingconfiguration of construction layers 381, 382 and 383 is repeated forevery three layers thereafter with every third layer having the samex-coordinates.

Layers 381, 382 and 383 are printed at a relatively low resolution of Ndpi, along the x-axis and corresponding x-pitch, “Δx”=1/N inches. Letthe pitch fraction be represented by “1/P” where “P” is a number greaterthan 1. Then the x-coordinates of droplets 391 in an “n-th” row ofdroplets printed by the RPA are (x_(O)+(n−1)Δx), where x_(O) is thex-coordinate of a first droplet along the x-axis in layer 381. Inaccordance with an embodiment of the invention, correspondingx-coordinates of droplets 392 in layer 382 are (x_(O)+(n−1)Δx+(1/P)Δx)and corresponding x-coordinates of droplets 393 in layer 383 areoptionally (x_(O)+(n−1)Δx+Δx).

The x-coordinates of droplets in each layer 381, 382 and 383 correspondto the x-coordinates of homologous voxels defined by the constructiondata of an object responsive to which the layers are printed. Theconstruction data voxels corresponding to droplets in adjacent layersare displaced relative to each other by a distance corresponding to(1/P)Δx.

In some embodiments of the invention, the droplets in each layercorrespond to homologous voxels in corresponding layers definedresponsive to the construction data that are partitioned into voxelshaving an x-pitch equal to Δx and corresponding to “low” printingresolution N. The layers are “thin layers” that have a thicknesscorresponding to that of construction layers 381, 382 and 383.

In some embodiments of the invention the droplets in all three layerscorrespond to homologous voxels in a “thick” layer defined responsive tothe construction data that has thickness corresponding to that of allthree layers combined. The thick “construction data” layer ispartitioned into voxels having an x-pitch (1/P)Δx that corresponds tothe high effective printing resolution (P×N). Droplets 391 inconstruction layer 381 correspond to those voxels in the constructiondata layer having x-coordinates corresponding (x_(O)+(n−1)Δx). Droplets392 and 393 in layers 382 and 383 correspond to those droplets in theconstruction data layer having x-coordinates corresponding to(x_(O)+(n−1)Δx+(1/P)Δx) and (x_(O)+(n−1)Δx+Δx) respectively.

The inventors have found that an effective printing resolution forlayers 281, 282 and 283 is substantially equal to P×N, corresponding toan effective x pitch equal to Δx/P. The effective x pitch Δx/P forlayers 281, 282 and 283 is indicated in FIG. 14.

By way of numerical example, P in FIG. 14 is equal to 2 and the pitchfraction is 0.5. If low printing resolution N is equal to 600 dpi alongthe x-axis and corresponding x pitch 1/600 in., then an effectiveprinting resolution along the x-axis for the layer is 2×600=1200 dpi anda corresponding high resolution x pitch is equal to 1/1200 in.

An RPA production facility for producing objects comprises one or moreof RPAs that define and provide a limited production capacity. Asrequests to produce objects are received by the facility, the facilitymust determine how to allocate and schedule its production capacity tomeet the demand. Each request for production defines at least one objectto be produced in accordance with a set of object specifications thatdefines a set of RPA operating parameters (e.g. resolution, layerthickness, waste ratio, production time . . . ) that is often differentfrom that of other production requests. Allocating and schedulingproduction capacity is therefore in general complicated and frequentlyrequires a production manager aided by appropriate computer programs tooversee and implement job scheduling.

In accordance with an embodiment of the invention, allocation andscheduling of production capacity of an RPA facility is performed by ajob management algorithm (JMA) that interfaces directly with a user andallocates and schedules production capacity, optionally, withoutintervention of a production manager.

When a user wants to place an order with the facility for a productionjob, the user accesses the JMA. The JMA in response presents the user,optionally on a computer screen using a suitable GUI, a plurality ofvirtual construction platforms. Each virtual construction platformrepresents a construction platform, such as construction platform 24shown in FIG. 1 on which an RPA of the RPA production facilityconstructs objects ordered from the facility.

Each virtual construction platform is characterized by a plurality of“platform parameters”. The platform parameters define, by way ofexample, an estimated time at which production of objects on theplatform is scheduled to begin and optionally end, available productionspace on the platform and cost of the space. Optionally, platformparameters comprise RPA operating parameters, such as layer thickness,resolution and waste ratio, in accordance with which, the RPA facilitywill produce objects on the platform.

The user chooses a platform having sufficient available space for theobject that the user wants to produce, RPA operating parameters thatcorrespond to the production job's object specifications and thatprovides him or her with suitable scheduling and cost. Once the usercompletes the process of choosing a platform, the user satisfiesadministrative requirements, such as arranging for payment or endorsinga purchase order, to finalize and reserve production space and time heor she has ordered.

In some embodiments of the invention, a user may define at least someplatform parameters of a platform. For example, the JMA optionallypresents the user with at least one “empty” construction platform forwhich the user can define platform parameters. Optionally, a platformparameter that the user can define comprises a priority, which, if sethigh enough, may enable the user's job to be produced out of turn,before other previously scheduled jobs. The JMA in response to platformparameters defined by the user optionally generates a corresponding costof production space on the platform. For example, if the user sets avery high priority for a platform that preempts scheduling of other jobsthe JMA determines cost of space on the platform accordingly.

In many situations, the user will not be in a position to convenientlydetermine RPA operating parameters, platform space, cost and otherparameters, hereinafter “job data” needed to execute the user's job. Forsuch cases, optionally the JMA offers the user a production wizard thataids the user in determining job data. Optionally, the wizard aids theuser via an interactive interrogation session in which the wizardpresents the user with questions whose answers are used to determine jobdata. Optionally, the user transmits construction data that define theobject that the user wants to produce and the wizard determines job datafrom the transmitted construction data. Once job data is defined, thewizard may highlight or otherwise indicate, which of a plurality ofproduction platforms are suitable for the user's job.

In the description and claims of the present application, each of theverbs, “comprise” “include” and “have”, and conjugates thereof, are usedto indicate that the object or objects of the verb are not necessarily acomplete listing of members, components, elements or parts of thesubject or subjects of the verb.

The present invention has been described using detailed descriptions ofembodiments thereof that are provided by way of example and are notintended to limit the scope of the invention. The described embodimentscomprise different features, not all of which are required in allembodiments of the invention. Some embodiments of the present inventionutilize only some of the features or possible combinations of thefeatures. Variations of embodiments of the present invention that aredescribed and embodiments of the present invention comprising differentcombinations of features noted in the described embodiments will occurto persons of the art. The scope of the invention is limited only by thefollowing claims.

1-59. (canceled)
 60. Apparatus for producing an object by sequentiallyforming thin layers of a construction material one on top of the otherresponsive to data defining the object, the apparatus comprising: amovable set comprising multiple printing heads, each having a surfaceformed with a plurality of output orifices offset from each other in afirst direction by a first distance, the plurality of printing heads ofthe set being placed in order in a second, scan, direction substantiallyperpendicular to the first direction; and a controller adapted to scanthe movable set with respect to a support surface in the second, scan,direction and to control the printing heads to dispense the constructionmaterial through each of their respective orifices responsive to thedata, such that each orifice dispenses material along a constructionline separated by said first distance from the line along which aneighboring orifice in a same head dispenses its material, wherein theeach of the heads that deposits construction material after the firsthead, deposits construction material along lines between the lines alongwhich construction material was previously deposited by the otherprinter heads.
 61. Apparatus according to claim 60 wherein each of theheads that deposits construction material after the first head, depositsconstruction material along lines equidistant from adjacent lines alongwhich construction material was previously deposited by the otherprinter heads.
 62. Apparatus according to claim 60 wherein the set ofprinter heads comprises a movable set of four printing heads, first,second, third and fourth printing heads of the set being placed in thatorder in the second, scan, direction, wherein the orifices in the secondprinting head are offset from those in the in the first printing head,in the first direction, by one half the first distance, such that theydispense the construction material along lines substantially equidistantfrom lines along which construction material is dispensed by the firstprinting head; and wherein the orifices in the third and fourth printingheads are so positioned that they dispense construction material alonglines equidistant from the adjacent lines along which material isdispensed by the first and second heads.
 63. Apparatus according toclaim 62 wherein the construction material is deposited in a first layerin accordance with the data in a first pixel matrix having a pitch, andin a second layer above the first layer, responsive to the data in asecond pixel matrix, wherein the first and second pixel matrices areoffset from each other in one or both of the first and second directionsby an amount that is less than the pitch.
 64. Apparatus for producing anobject by sequentially forming thin layers of a construction materialone on top of the other responsive to data defining the object, theapparatus comprising: at least one printing head, having a surfaceformed with a plurality of output orifices offset from each other in afirst direction by a first distance; and a controller adapted to scanthe at least one printing head with respect to a support surface in thesecond, scan, direction perpendicular to the first direction and tocontrol the printing heads to dispense the construction material througheach of their respective orifices responsive to the data, such that eachorifice dispenses material in accordance with a first pixel matrixhaving a pitch to form a first layer of patterned construction material;and then to scan the at least one printing head with respect to thesupport surface in the scan direction to form a second layer above thefirst layer, responsive to the data in a second pixel matrix, whereinthe first and second pixel matrices are offset from each other in one orboth of the first and second directions by an amount that is less thanthe pitch.
 65. Apparatus according to claim 64 wherein, while formingthe second layer, the at least one printing head is shifted in firstdirection by a whole number of pitches plus said offset from itsposition in forming the first layer
 66. Apparatus according to claim 64wherein the offset is one half the pitch.
 67. Apparatus according toclaim 64 wherein the first and second pixel matrices are offset fromeach other in the second direction.
 68. Apparatus according to claim 60wherein the printing heads are mounted in a shuttle and wherein eachprinting head comprises at least one registration structure that matchesa registration structure comprised in the shuttle and when a printinghead is mounted to the shuttle its at least one registration structurecontacts the corresponding shuttle registration structure and positionsthe printing head accurately in the shuttle.
 69. Apparatus according toclaim 68 wherein the at least one registration structure comprised ineach printing head and its corresponding shuttle registration structureposition the printing heads so that their respective lines of orificesare parallel.
 70. Apparatus according to claim 68 wherein the at leastone registration structure comprised in each printing head and itscorresponding shuttle registration structure, position the printingheads so that the first orifice in each printing head is accuratelypositioned relative to the first orifices of the other printing heads.71. Apparatus according to claim 60 wherein the construction materialcomprises a photopolymer.
 72. Apparatus according to claim 71 andcomprising a lamp that provides radiation to polymerize thephotopolymer.
 73. Apparatus for producing an object by sequentiallyforming thin layers of a material one on top of the other responsive todata defining the object, the apparatus comprising: at least oneprinting head having a surface formed with at least one output orificeand controllable to dispense a photopolymer material in liquid formthrough the orifice; a wiper; and a controller adapted to control theprinting head to dispense the photopolymer and sequentially form thelayers, and to move the printing head over the wiper to clean thesurface in which the orifices are formed, wherein the cleaning blade isformed with at least one slot that partitions the cleaning blade into aplurality of teeth each having an edge that contacts an orifice surfaceof a different one of the plurality of printing heads and scrapes excessconstruction material form the surface.
 74. Apparatus according to claim73 wherein the wiper comprises at least one cleaning blade having anedge that scrapes excess construction material from the surface when thecontroller controls the surface to move over the wiper.
 75. Apparatusaccording to claim 73 wherein the cleaning blade is formed from aresilient material so that the edge that contacts the surfacesresiliently contacts the surface.
 76. Apparatus according to claim 75wherein the edge is scalloped and has a different scallop correspondingto each printing head of the at least one printing head.
 77. Apparatusaccording to claim 73 wherein the at least one cleaning blade comprisesat least two cleaning blades.
 78. Apparatus according to claim 77wherein a cleaning blade of the at least two cleaning blades has an edgethat does not contact the orifice surface of a printing head but movesalong and in close proximity to the surface when the controller controlsthe surface to move over the wiper.
 79. Apparatus according to claim 60and comprising an obstacle detection system that detects defects in alayer that protrudes from a surface of the layer.
 80. Apparatusaccording to claim 79 wherein the obstacle detection system comprises: alaser that provides a laser beam that contacts or is located close tothe surface of the layer along a length of the laser beam; a detectorthat receives light from the laser beam; and wherein light that thedetector receives from the laser is at least partially blocked by adefect that protrudes from the surface.
 81. Apparatus according to claim65 wherein the offset is one half the pitch.
 82. Apparatus according toclaim 64 wherein the printing heads are mounted in a shuttle and whereineach printing head comprises at least one registration structure thatmatches a registration structure comprised in the shuttle and when aprinting head is mounted to the shuttle its at least one registrationstructure contacts the corresponding shuttle registration structure andpositions the printing head accurately in the shuttle.
 83. Apparatusaccording to claim 82 wherein the at least one registration structurecomprised in each printing head and its corresponding shuttleregistration structure position the printing heads so that theirrespective lines of orifices are parallel.
 84. Apparatus according toclaim 82 wherein the at least one registration structure comprised ineach printing head and its corresponding shuttle registration structure,position the printing heads so that the first orifice in each printinghead is accurately positioned relative to the first orifices of theother printing heads.
 85. Apparatus according to claim 64 wherein theconstruction material comprises a photopolymer.
 86. Apparatus accordingto claim 85 and comprising a lamp that provides radiation to polymerizethe photopolymer.
 87. Apparatus according to claim 64 and comprising anobstacle detection system that detects defects in a layer that protrudesfrom a surface of the layer.
 88. Apparatus according to claim 73 andcomprising an obstacle detection system that detects defects in a layerthat protrudes from a surface of the layer.
 89. Apparatus according toclaim 87 wherein the obstacle detection system comprises: a laser thatprovides a laser beam that contacts or is located close to the surfaceof the layer along a length of the laser beam; a detector that receiveslight from the laser beam; and wherein light that the detector receivesfrom the laser is at least partially blocked by a defect that protrudesfrom the surface.
 90. Apparatus according to claim 88 wherein theobstacle detection system comprises: a laser that provides a laser beamthat contacts or is located close to the surface of the layer along alength of the laser beam; a detector that receives light from the laserbeam; and wherein light that the detector receives from the laser is atleast partially blocked by a defect that protrudes from the surface.